HLA-DMA and -DMB genes are both required for MHC class II/peptide complex formation in antigen-presenting cells

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Abstract
MAJOR histocompatibility complex (MHC) class II molecules are highly polymorphic cell-surface glycoproteins that present antigenic peptides to CD4+ T lymphocytes. The normal assembly of class II molecules with cognate peptides for antigen presentation requires an accessory function provided by a gene mapping to the class II region of the HLA complex1,2. The isolation of somatic cell mutants of antigen-presenting cells (APC) has shown that at least one gene which maps between HLA-DP and HLA-DQ, provisionally designated c2p-1 (ref. 3), mediates this process1,2,4. Here we describe a unique new mutant 2.2.93, which manifests defective formation of class II/peptide complexes like that described in c2p-1 mutants. We show that (1) mutant 2.2.93 contains a mutation in HLA-DMA5, and a representative c2p-1 mutant, 9.5.3, contains a mutation in HLA-DMB5; and (2) transfection and expression of DMA complementary DNA in 2.2.93, and DMB cDNA in 9.5.3, reverses their mutant phenotypes. These results show that HLA-DMA and -DMB, genes of previously unknown function mapping between HLA-DP and HLA-DQ5, are required for the normal assembly of peptides with MHC class II molecules. They suggest that HLA-DMA and -DMB encode subunits of a functional heterodimer which is critical in the pathway of class II antigen presentation.